As it is well-known, a voltage controlled oscillator (hereinafter, referred to as a VCO) is widely used in a modulator in a wireless radio as a device for generating local oscillation signals. Frequency modulation signals and phase modulation signals can be generated by using the VCO. Furthermore, modulation signals (phase shift keying (PSK), code division multiplex access (CDMA), orthogonal frequency division multiplex (OFDM), etc.) that also have modulation components in amplitude components can be generated, by controlling the voltage supplied to a power amplifier while having constant envelope modulation signals generated by the VCO to be inputted into the power amplifier. In recent years, in order to adapt such a modulation device to a communication system that uses multiple different frequency bands, it is necessary to adjust oscillation frequencies of the VCO in a broad frequency range.
Therefore, in order to achieve broadening of bandwidth for a modulation device, a modulation method called two-point modulation has been proposed. FIG. 11 shows a configuration example of a modulation device that uses a conventional two-point modulation method.
In FIG. 11, a conventional two-point modulation device 501 includes a calculation section 521, a frequency error calculation section 522, a loop filter 523, an addition section 525, a VCO 526, a frequency detection section 527, and a buffer 528.
A modulation signal is converted at the calculation section 521 into a signal that corresponds to a desired frequency channel, and is outputted as a low pass response signal via the frequency error calculation section 522 and the loop filter 523. Furthermore, a modulation signal is adjusted to be a signal that is necessary for the buffer 528, and is outputted as a high pass response signal. The addition section 525 adds the low pass response signal and the high pass response signal, and inputs the result into the VCO 526. A signal outputted by the VCO 526 is inputted, as a feedback, into the frequency error calculation section 522 via the frequency detection section 527. The frequency error calculation section 522 detects and outputs a frequency error between a modulation signal outputted from the calculation section 521 and a signal outputted from the frequency detection section 527. With such feedback processing, the frequency of the signal outputted by the VCO 526 becomes stable.
As described above, by using the two-point modulation method, broadening of the bandwidth for a modulation device can be achieved (FIG. 12) since a modulation characteristic derived through a combination of a frequency gain that is a low pass response passing through the feedback circuit and a frequency gain that is a high pass response passing through the feed-forward circuit can be obtained.
However, even when this two-point modulation method is employed, if a nonlinear VCO 526 is used, a problem arises where a frequency characteristic having a broad band cannot be obtained since distortion occurs in the output due to having a narrow frequency band in which linear modulation can be conducted (FIG. 13). Therefore, it becomes necessary to calibrate the nonlinearity and gain of the VCO 526.
An invention disclosed in Patent Literature 1 exists as one that solves the above described problem. FIG. 14 shows a configuration example of a conventional direct modulation device 511 described in Patent Literature 1.
In FIG. 14, the conventional direct modulation device 511 includes a PLL circuit comprising a VCO 1506, an N-divider (N counter) 1508, a phase comparator, a charge pump (CP), and an RC coupling filter. A phase signal corresponding to a desired channel is converted into a digital modulation signal by a ΔΣ modulator and supplied to the N-divider 1508 and the phase comparator. A step signal ΔfPM is converted into an analog signal by a D/A converter 1510, and then inputted into an auxiliary terminal 1504 of the VCO 1506 via a low pass filter (hereinafter, referred to as LPF) 1512.
In this configuration, the PLL circuit is operated in a closed loop state. First, a desired channel frequency fc is inputted to lock up the VCO 1506 at a division ratio N, and a lock-up voltage Vctrl at that moment is retained [fREF=fC/N]. Next, a step signal ΔfPM is inputted to shift the division ratio of the N-divider 1508 by ΔN. Then, the step signal ΔfPM is adjusted [fREF=(fC+ΔfPM)/(N+ΔN)] such that the lock-up voltage Vctrl at this moment becomes identical to the initial lock-up voltage. The conventional direct modulation device 511 calibrates the nonlinearity and gain of the VCO 1506 by conducting such processes at multiple calibration points.
CITATION LIST